A common type and widely used variable speed belt drive consists of a pair of sheaves and a belt interconnecting the sheaves in which one of the flanges on at least one of the sheaves is mounted on an axially moveable hub and is adjustable toward and away from the other flange. The hub is mounted on a shaft, and a keying device, such as a key and key ways, interconnects the flange, hub and shaft and prevents relative rotation therebetween. The adjustable sheave is varied by an operator or in response to a control signal and the other sheave automatically adjusts itself to accommodate the adjustment made in the first sheave. In a modification of this basic type, one of the sheaves is variable and the other is moved relative to the adjustable sheave to compensate for variations in the effective belt length when the adjustment in the speed is made. These variable speed drives often operate for extended periods of time under heavy loads, and are constantly subjected to excessive wear, particularly in the axially slidable sheave parts involved in the adjustment for different speeds. As a result of these prolonged adverse operating conditions, notching of the keying device and a substantial amount of fretting and wear occur in the parts subjected to the sliding contact, ultimately interfering with the proper operation and adjustment of the sheaves and necessitating the eventual replacement of the affected parts or of the complete sheave. The sliding parts of the sheave normally most affected by the fretting and wear under the aforementioned conditions consist of the shaft of the drive sheave and the hub slidable thereon, the hub usually being keyed to the shaft for rotation therewith and, when in proper operating condition, being freely moveable axially on the shaft. However, adverse operating conditions may cause roughness and binding between the key in the shaft and the contacting sliding portion of the hub, so that the sliding movement between the two parts is restrained to the extent that optimum operation of the variable pitch sheave is not possible even under normal operating conditions.
Various types of mechanisms and systems have been used for adjusting the moveable flange of the variable pitch sheave or sheaves. These include mechanical types, manually operated or power driven, or hydraulic types. In both the mechanical and hydraulic types of prior adjustment mechanisms, the moveable flange is keyed to the shaft and is adapted to move axially on the shaft in response to changes in the setting on the adjustment mechanism therefor. These adjustment mechanisms and systems function satisfactorily under normal operating conditions while the drives are in good condition, but they often become unreliable or inoperable from fretting and scoring of the shaft and keys. In order to overcome this difficulty, plastic bushings disposed between the shaft and the hub of the moveable flange, and in some instances along the keys, have been used; however, the basic reason for the adverse condition developing along the keys is not eliminated by these bushings, although the problem may be somewhat alleviated. Further, the hydraulic system for adjusting the moveable flange is often relatively complicated and difficult to maintain in optimum operating condition.
In conventional variable speed belt drives having a fixed flange in the sheave, the flange is secured to a shaft which is connected directly to and supported by the drive motor shaft, or which is journaled in a pillow block at each end and which extends from one of the pillow blocks for connection to the motor shaft. The sheave and motor shafts may be connected by a flexible coupling having flanges with hubs for mounting on the two shafts in spaced end-to-end relation. This type of shaft connection requires a substantial amount of space in order to assemble the two shafts and the coupling beyond the respective pillow block, thus making the overall size of the drive relatively large and at times too large for particular installations, wherein compactness is essential to fit in limited areas, such as that available in air conditioner housings and the like. The direct coupled type of drive described above has the disadvantage of being extremely difficult to align. The motor-mounted constant-speed shaft and the separately mounted variable speed shaft must be made parallel and square (belt between the sheaves) in three planes to a very small tolerance, after the pieces are installed in the machine to be driven. Since sophisticated alignment devices, expertise and the time required to make the proper alignment, are seldom available at an installation site, all independent-shaft belt connected mechanical variable speed drives basically have a short belt and sheave life caused by misalignment.